Integrated circuit fabrication is typically accomplished by forming many different layers on a substrate. As used herein, the phrase integrated circuit refers to circuits such as those formed on monolithic substrates of a semiconducting material, such as group IV materials like silicon and germanium, and group III–V compounds such as gallium arsenide. Because the design tolerances of an integrated circuit are so strict, it is desirable to monitor the properties, such as thickness and elemental composition, of the various layers as they are formed. One way to measure the properties of film layers is to use electron microprobe x-ray spectrometry.
Electron microprobe x-ray spectrometry uses an electron beam source to excite a sample. X-rays having wavelengths that are characteristic of the elements of the sample are emitted from the sample over a continuous range of takeoff angles, defined as the angle between the x-ray and the sample surface. An x-ray detector assembly is positioned to detect a fraction of the x-rays that are emitted from the sample. The detector assembly can capture x-rays emitted over a finite range of takeoff angles. The detector assembly includes both a spectrometer and an x-ray detector. The spectrometer selects x-rays within a narrow range of wavelengths and directs only those x-rays to the x-ray detector. This is typically accomplished by rotating a diffractor through a range of angles, where at each angular position of the diffractor, the diffractor deflects x-rays with a given wavelength range towards the detector. The rate of impingement of the x-rays within subsets of the desired range of wavelengths is sequentially detected and measured. From this information, properties such as the elemental composition and thickness of the sample can eventually be determined.
The x-ray detector assembly collects x-rays over a finite range of takeoff angles and counts them as an aggregated unit. For a given electron beam energy, data collected from each element in the sample consists of a single number, the x-ray counts per second within the characteristic wavelength range for that element. Since there are usually only two or three elements present, the entire data set likewise consists of only two or three numbers. This is just barely a sufficient amount of data to determine the thickness and the composition of the sample, as there are typically many different variables that are confounded within this data. There is no redundancy in the data that can be used to check for inconsistencies or departures from the mathematical model that is used to analyze the data. Additional data can be obtained by changing the electron beam energy and counting the x-rays again, but this is time consuming and requires more complex electron optics and control electronics.
Because electron microprobe x-ray spectrometry can provide information in a nondestructive manner, it would appear that x-ray spectrometry would be an excellent tool for use in the integrated circuit fabrication industry. Unfortunately, electron microprobe x-ray spectrometry tends to be too slow for use as an in-process tool because of the slow scanning process that must be performed, as introduced above. Not only must a spectrometer be scanned through the desire range of wavelengths, but each scan position must be held for a given length of time so that a sufficient amount of x-rays can be collected to give a valid reading. Further, if the energy of the sample excitation is changed to produce additional, confirmatory information, then the entire scanning process must be performed again as many times as necessary. This tends to make electron microprobe x-ray spectrometry too time consuming for use as an in-process measurement tool.
What is needed, therefore, is an x-ray spectrometer that overcomes some of the problems described above.